Quasielastic light scattering ("QLS"), also known as photon correlation spectroscopy, is a technique which, in certain instances, can be used to determine various physical characteristics of a particulate suspended in a liquid and/or the nature of the suspension itself. See Madani and Kaler (Part. Syst. Charact., Vol. 8, pp. 259-266, (1991)), Berne and Pecora ("Dynamic Light Scattering," John Wiley, New York, 1976) and D. H. Everett, "Basic Principles of Colloid Science," Royal Society of Chemistry, London 1988). QLS is based on determining the velocity distribution or particle movement by measuring dynamic fluctuations of intensity of scattered light. The disperse particles of macromolecules are suspended in a liquid medium undergo Browning motion which causes fluctuations of the local concentration of the particles. This results in local inhomogeneities of the refractive index which in turn results in fluctuations of intensity of the scattered light. The linewidth of the light scattered spectrum (defined as the half-width at half-maximum) is proportional to the diffusion coefficient of the particles.
However, this technique cannot be applied to all suspensions. Certain suspensions, because of the concentration of the particles, the size and/or size distribution of the particles, the type of particles, the possibility of interparticulate interactions, the type of media and the overall light absorption properties of the system, are not suitable for analysis using this technique.
U.S. Pat. No. 5,624,914 to Patel et al., the text of which is incorporated by reference, discloses certain lipophilic oligosaccharide antibiotics in specific physical forms, as well as formulations which establish or maintain the drug in those forms. A reliable and accurate method of confirming the form(s) of those active drugs in suspension would be highly desirable. A technique which could also confirm the effects on the suspension of various suspending agents in various proportions would also be useful. QLS has not previously been used for characterizing antibiotic-containing suspensions and in particular, suspensions of lipophilic oligosaccharide antibiotics and salts thereof.